Directional hearing aid with variable directivity

ABSTRACT

A directional hearing aid to be fitted to a person&#39;&#39;s head and wherein the housing has front and rear openings to receive sounds from the front and rear, a directional microphone in the housing respectively receiving the sounds from front and rear, and a variable acoustic resistance in one of the openings of the housing, the acoustic resistance comprising a grill entirely filling the opening and having minute soundreceiving apertures, and a rotary plate having partly open areas to variably open and close the apertures of the grill for changing the acoustic resistance and changing the directivity effect of the directional hearing aid.

United States Patent [191 Moen I 1 DIRECTIONAL HEARING AID WITH VARIABLEDIRECTIVITY [75] Inventor: Bruce A. Moen, Maple Grove,

Minn.

[73] Assignee: Textron, Inc., Providence. R.1.

[22] Filed: Jan. 2, 1973 [21] Appl. No.: 320,072

[52] U.S. Cl. 179/107 FD; 179/107 H [51] Int. Cl. H04r 25/00 [58] Fieldof Search 179/107 FD, 107 E, 107 R.

179/107 H, 1 DM 3.770.911 11/1973 Knowles 179/107 S PrimaryExaminerRalph D. Blakeslee Attorney, Agent, or FirmH. Dale Palmatier 57ABSTRACT A directional hearing aid to be fitted to a persons head andwherein the housing has front and rear openings to receive sounds fromthe front and rear, a directional microphone in the housing respectivelyreceiving the sounds from front and rear, and a variable acousticresistance in one of the openings of the housing, the acousticresistance comprising a grill entirely filling the opening and havingminute soundreceiving apertures, and a rotary plate having partly openareas to variably open and close the apertures of the grill for changingthe acoustic resistance and changing the directivity effect of thedirectional hearing aid.

13 Claims, 7 Drawing Figures DIRECTIONAL HEARING AID WITH VARIABLEDIRECTIVITY I BACKGROUND OF THE INVENTION .In a directional hearing aidof the type disclosed in U.S. Reissue Patent RE 27,487, it is importantthat sound be admitted at both the front and rear of the housing so asto supply soundto both, sides of the diaphragm in the microphone.Ordinarily, an acoustic resistance is incorporated at the rear of themicrophone so as to introduce a time delay or phase shift in thetransmission of sound waves to the diaphragm which enter the rear of themicrophone, as compared to those which enter, unimpeded, at the front ofthe microphone. Thus, as for sounds which originate in front of themicrophone, such a phase shift causes sound waves entering the rear ofthe microphone to have an additive effect in moving the diaphragm alongwith the sound waves entering the microphone from the front. Thisadditive effect is in contrast to the neutralizing effect of sound wavesoriginating from the rear and reaching the opposite sides of thediaphragm simultaneously to thereby oppose each other and prevent thediaphragm from moving. Of course, this additive effect is transformedinto a larger magnitude of signal from the microphone.

The microphone with the acoustic resistance at the rear of themicrophone responds to sound originating from a location to the rear ofthe microphone so as to produce more of a neutralizing effect as thesound waves are applied at opposite sides of the diaphragm so that thediaphragm has less magnitude of movement and generates a lessermagnitude of signals for output from the microphone than in the case ofsounds which originate from in front of the microphone.

It has been found that the amount of the resistance at the rear of themicrophone is rather critical, and that the directional response of themicrophone and the hearing aid in which the microphone is incorporatedwill peak at a rather small range of acoustic resistance, and if theacoustic resistance is either increased or decreased out of such a smallrange, the-directional characteristics of the microphone and hearing aidare considerably reduced.

It has also been determined that the relative magnitude of the acousticresistance at the rear of the microphone to the resistance at the frontof the microphone at the openings into the microphone for admittingsound waves to opposite sides of the diaphragm is important. As aresult, it may be desirable in certain instances to apply an acousticresistance'in relation to the opening at the front of the microphone, aswell assome resistance in relation to the opening atthe rear of themicrophone so that the combination of.acoustic resistances is such as tomaximize the directional effect of the microphone and the ability ofthe. microphone to maximize its signal response on soundswhich originatefrom a location in front of the microphoneas compared to the signalresponse on sounds which originate from the rear of the microphone.

In setting the acoustic resistances in relation to a miduce all of theparts and their relationships and exactly reproduce the acousticresistances in each microphone and hearing aid and make them the same asin all other of these tiny microphones and hearing aids. It is also tobe recognized that various persons wearing hearing aids can tolerate ordesire various degrees of directivity in the hearing aid.

. "S M ARY OF THE INVENTION out regard to the hearing aid housing. Anadditonal acoustic resistance with minute clear passages is provided inthe ports of the hearing aidhousing through which the sound waves arepermitted to pass to the front and rear of the microphone. v

In many cases, the variable acoustic resistance is disposed in thepassage of the hearing aid housing which supplies sound waves to therear of the microphone, but, in order to change the combination ofacoustic resistances which are effective to change the directivityeffect of the unit, a variable acoustic resistance may be establishedadjacent the front of the microphone.

Use of these adjustable acoustic resistanceswill vary the combination ofresistances in the directional hearing aid so that the hearing aid canbe tuned to itsmaximum directional characteristic or to such otherdirectional characteristics as may be comfortable to or desired by thewearer of the hearing aid.

The acoustic resistances-may be controllable from the exterior of thehearing aid housing, or may be controllable or variable only-- byobtaining access into the interior of the hear-ing aid housing.Ordinarily, the wearer of the hearing aid does not have access to theinterior of the hearing aid housing but technicians, who are familiarwith hearing aid features; fInd it very useful tobe able to tune adirectional hearingaid to its maximum directivity and this may beaccomplished by a variable acoustic resistance confined wholly withinthe housing of the hearing aid. An additional variable acousticresistance may be provided for reducing the directional effect of thehearing aid unit and may be controllable or adjustable by the wearer ofthe hearing aid or by a person who. is capable of fitting the hearingaid to the wearer.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an elevation view of abehind theear hear-- ing aid which incorporates the present invention.

FIG. 2 is an elevation view taken at an oblique angle as indicated at2-2 in FIG. 1 showing the rear detail of the hearing aid housing.

FIG. 3 is an enlarged detail section view taken approximately at 3-3 inFIG. 2.

F IG. 4 is an exploded perspective view of certain important partsof thevariable acoustic resistance of the hearing aid. 7 v

FIG. 5 is an enlarged detail section view somewhat similar to FIG. 3 andshowing a modified form of the invention.

FIG. 6 is a greatly enlarged detail section view somewhat similar toFIG. 3 and showing an additional modified form of the invention.

FIG. 7 is a response curve showing the gain in output signal produced bythe hearing aid illustrated in FIGS. 1-4; curve F shows the gainproduced by the hearing aid in relation to sounds originating at alocation in front of the hearing aid; and curves R R and R show I theresponse characteristics to sounds originating at a location to the rearof the hearing aid, and wherein curve R, illustrates the response in thecondition wherein the variable acoustic resistance is at a minimum, andcurve R illustrating the response in the condition wherein the variableacoustic resistance is at nearly a maximum, and curve R illustrates theresponse of the hearing aid in the condition wherein the variableacoustic resistance is of intermediate magnitude.

DETAILED DESCRIPTION OF THE INVENTION The form of the inventionillustrated in FIGS. 1-4 is a behind-the-ear hearing aid, indicated ingeneral by numeral 10 and having a housing 11 which confines all of thehearing aid components including the microphone 12 as well as thecircuitry, the battery, the sound producing transducer from which soundis transmitted through the nipple 12 to which the sound tube 13 and earmold 14 are connected. The volume of the output may be regulated by thevolume control 15.

The hearing aid housing 11 defines an isolated microphone chamber 16which is separated from the other portions of the interior of housing 11by a full wall 17 which minimizes any transmission of soundstherethrough. Sounds are permitted to enter the front and rear portionsof the microphone chamber 16 through access ports 18 and 19 formed inthe front and rear wall portions 20 and 21, respectively, of the housing11 which defines the microphone chamber 16. Thefront port 18 has aprotective grill 22 secured therein as by adhesive or otherwise toprotect the microphone 12 and prevent foreign materials or articles fromentering or obstructing the access port or opening 18. The apertures22.1 in the grill 22 are sufficiently large and numerous as tocumulatively provide open space which is essentially non-resistive totransmittal of sound therethrough.

The microphone 12 is confined in the chamber 16 and physically locatedby a soft and resilient spacer pad 23, which underlies the microphoneand has upright ends 23.1 to cushion the ends of the microphone. Pad 23has a belt portion 24 surrounding the girth area of the microphone l2.Pad 23 including the integral belt portion is constructed, of soft andresilient sound absorbing material such as soft porous rubber or plasticso as tosubstantially completely isolate the front portion 16.1 of themicrophone chamber from the rear portion 16.2 thereof and therebyprevent any migration of sound around the exterior of the microphone andbetween the front and rear portions of the microphone chamber. The pad23 with belt 24 of resilient material also confines the microphone 12against movement in the chamber 16 because the pad and belt engage boththe case 12.1 of the microphone and the adjoining wall areas 17 and 11.1of the hearing aid housing.

The microphone 12 has a diaphragm 12.2 and a transducer 12.3 which isoperated by the diaphgram. The diaphragm 12.2 separates the microphonecase into two separate compartments so that sounds'entering bothcompartments will apply pressures against the diaphragm so as to causeoscillation or vibration thereof. The upper compartment in the hearingaid case communicates with a soundqeceiving opening 12.4 disposed at thefront portion of the microphone and specifically in the front wall 12.5thereof. In this form, the sound-receiving opening 12.4 is essentiallynon-restrictive or non-resistive to passage of sounds therethrough sothat the sounds and sound pressure variations may enter the uppercompartment of the microphone substantially unimpeded. The uppercompartment of the microphone will thereby receive sounds from the frontport 18 of the hearing aid housmg.

The lower compartment of the microphone 12 has one or moresound-receiving openings 12.6 located adjacent the rear portion of themicrophone case 12.1 at a location below the diaphragm 12.2. In the formshown, the openings 12.6 are minute in size and are formed in a separateplate 12.7 secured over a large opening 12.8 formed in the rear portionof the hearing aid case at a location below the diaphragm 12.2. Theopenings 12.6 are extremely small and may be formed by etching orsimilar processes so that the size of the openings 12.6 form anacoustical resistance between the interior of the microphone and therear portion 16.2 of the microphone chamber. As previously described,the resistance provided by the openings 12.6 to passage of sounds intothe microphone cause a phase shift or delay in transmission of thesesounds relative to the transmission of sounds into the front portion ofthe microphone.

The amount of resistance provided by the openings 12.6 into themicrophone case is rather critical. The directivity of the microphone l2and of the hearing aid will be at a maximum for a rather narrow range ofresistances provided by the minute openings 12.6, and if the amount ofresistance provided for entry of sound into the rear portion of themicrophone is varied from such a small range, by either increasing theresistance or decreasing the resistance, the directivity effect of themicrophone and hearing aid will be reduced considerably.

Because of the criticality of the amount of resistance as it affects thedirectivity of the unit, it may be desirable to tune each hearing aid tothe maximum directivity, or at least to the level of directivity whichis desired.

The hearing aid 10 is provided with a variable acoustic resistance 25 inthe access opening or port 19 in the rear wall 21 of the hearing aidhousing. The variable acoustic resistance includes a grill or screen 26having a plurality of minute apertures or clear passages 26.1 therein(which are illustrated disproportionately large in the drawing as to bevisible) for permitting passage of sound therethrough. The apertures26.1 cumulatively define an open area through the grill 26 which hasessentially no resistance to transmission of sounds and sound wavestherethrough, but the apertures 26.1 are sufficiently small as to definea significant resistance to transmission of sounds therethrough whenmost of the apertures 26.1 are closed or covered and only a few of theapertures remain open.

The resistance 25 in combination with the volume of chamber 16.2 producethe phase shift or delay of sounds entering the rear port 19 of thehearing aid. The space in chamber 16.2 is fixed and acts in a mannersimilar to a capacitance in electrical circuitry. Adjusting theresistance 25 effects tuning so as to maximize, or vary, the phase shiftof sounds entering the rear port 19. Similarly, the resistance providedby minute openings 12.6, in combination with the space within themicrophone case 12.1 and beneath the diaphragm produces a phase shift ordelay of'sounds entering themicrophone from the time such sounds arefirst presented at the openings 12.6 and until such sounds are effectiveto produce an effect upon the diaphragm.

The variable acoustic resistance also includes a substantially circulardisc or masking plate 27 having a cylindrical hub 27.1 which fits snuglyinto the bearing opening 26.4 of the grill plate and is rotatable in theopening. The masking plate 27 will lie flush against the grill plate 26which is sealed around its entire periph eral edge 26.5 to the rear wallofthe microphone chamber. The masking plate or disc 27 is disposed in aslot 21.1 in the rear wall of the housing and the upper portion of thedisc 27 protrudes outwardly through an open side 21.2 of the slot.

The masking disc or plate 27 has a pair of openings 27.2 formed thereinand also a pair of adjacent closed areas 27.3 separated from the openareas by substantially radial edges 27.4.

The boundary 26.2'of area of apertures 26.1 in the grill is similar inshape to opening 27.2 of wheel 27 so that the resistance can bemaximized by closing all of openings 26.1.

The masking plate or disc 27 may be rotated by manual manipulation ofthe edge which protrudes through the side of slot 21.2, and the plate 27will progressively cover or uncover portions of the apertured areas ofthe grill 26 so as to progressively increase or decrease the resistanceto transmission of sound to the rear portion of the microphone.

The curves in FIG. 7 illustrate the operating characteristics of thehearing aid 10 at various adjustments of the rotary masking plate ordisc 27 of the variable acoustic resistance. The microphone 12 of thehearing aid 10 was turned so that as relates to sounds which originateat a location in front of the hearing aid housing, the gain recorded atthe output of the hearing aid in such sounds was at a maximum asindicated by the curve F in FIG. 7. Subsequently, the hearing aid wasreversed so that the sound was originating at a location to the rearofthe hearing aid, closest the access port 19. With the masking plate 27in the adjustment illustrated in FIG. 2 wherein the maximum number ofopenings 26.] of the grill are open and therefore the least resistanceto transmission of sounds is encountered, the gain produced by thehearing aid was recorded at the curve R At 1000 Hz. the gain with thesound originating at the front of the hearing aid was approximately 50dB, and the gain with the sound originating to the rear of the hearingaid was approximately 23 dB, and therefore the differential front torear gain was'27 dB, which indicates that, at this setting, the hearingaid had a maximum directivity effect. In other words, the hearing aidwas most sensitive to sounds originating at a location infront of thehearing aid and least sensitive to sounds originating at the rear of thehearing aid.

The resistance produced by the variable acoustic resistance 25 wasthereafter increased at several different;

settings as depicted by the characteristics recordedin curves R and RThe differential gain, front to rear, between the curves R and F wasonly moderate, and

at v1000 Hz., the. differential gain, front to rear, was 15 dB, stillsignificant, but ofmoderate magnitude.

With the .maskingplate 2-7 rotated so that substantially all ,oftheapertures in grill -26 are closed, the curve R was recorded. with thesound originating from a location to the rear of thehearingaid, and itwill be seen that the leastdifferential gain, front to rear, wasrecorded for this settingof the masking -plate27. In this condition,onlya 6dB differential gain was determined at 1000 Hz. l

It will be noted in the curves of FIG. 7, that curves R and R are mergedwith curves R and F and that the overall effect of the-variable acousticresistance 25 is nullified as the frequencies. approach 5000 Hz.

However, in the normal voice ranges, in the vicinity of 500 to 2000 Hz.,the effect of the variable acoustic resistance 25 is pronounced.

With the original tuning of microphone 12, the adding of additionalacoustic resistance by thevariable acoustic resistance 25 tends todecrease the directivity effect of the hearing aid. However, themicrophone 12 may have been tuned sothat the acoustic resistanceprovided by the minute clear passages 12.6 into the rear portion of thehousing, may have produced slightly less than maximum directivity of thehearing aid, whereupon the variable acoustic resistance 25 may havefirst caused the differential gain to increase and then subsequentlydecreaseas the variableacoustic resistance 25 producesmore resistance byclosing more and more of the apertures 26.1. I

In the form of the invention illustrated in FIG. 5,-the hearing aid 30has a housing 31 very nearly identical to the housing previouslydescribed. The microphone chamber 36 confines the microphone 32 and theresil ient belt 34 isolates the front and rear portions of themicrophone chamber from each other. The port 39 in the rear wall of thehousing has a simple protective grill 39.1 to protect the microphonefrom exterior influences, but to freely transmit,sounds'therethroughwithout significant resistance. The microphone 32 has a forwardly facingsound-receiving opening 32.1 of sufficient size as to be non-resistiveto transmission of sounds therethrough. The sound-receiving openings32.2 in the rear portion of the microphone and commu: nicating with thelower microphone compartment beneath the diaphragm are resistive totransmission of sounds. v I 1 An apertured grill 26, substantiallyidentical to the grill 26 described in connection with FIGS. 1-4 is secured about its entire periphery to the hearing, aid housing in thefront port 38 with the capability of in-. creasing the resistance tosounds transmitted to the front of the microphone if a number of theapertures in the grill 26' are closed or covered. This resistance incombination-with thechamber 16.1, produces a phase shift or delay ofthesound before its effects movement of the diaphragm. IArotatable disc ormasking plate 27' is rotatably mounted on and lies flush against theapertured grill 26' in the sameidentical fashion as is the grill 27related to the grill 26 in the form of the invention illustrated inFIGS. l-4..;Theopenings in the masking plate ordisc 27' may partiallycover and uncover the apertures in thegrill 26' as-the plate 27 isrevolved.

entering through the front of the hearing aid as well as those enteringinto the rear of the microphone.

In FIG. 6, the form ofthe hearing aid 10" is substantially identical tothat illustrated in FIGS. l-4 with certain exceptions. The microphone 12is the same and is located essentially identically to that previouslydescribed. The-rear wall 21' of the hearing aid housing partly ispartially defined by a cover 21 in this form of the invention, and it isthe cover 21 of the rear wall through which the rear access opening 19is defined. The rotatable masking disc 27 and the apertured grill 26 areboth identical to that illustrated in FIGS. 1-4 and are related to eachother in the same manner and function in the same manner as previouslydescribed.

In this form of the invention illustrated in FIG. 6, the access port hasan interior portion 19' thereof in which an additional variable acousticresistance 25' is located. The variable acoustic resistance 25' has anapertured grill 26.1, and a partly open rotary masking plate or disc27.1 both of which are substantially identical to the grill and maskingdisc 26, 27, respectively, but of slightly smaller dimensions. Inaddition, the rotatable masking plate 27.1 has an operating control lug40 located at its center and provided with a screw driver slot so thatthe disc 27.1 can be adjusted to produce the directivity of the hearingaid 10', and then a spot of adhesive 41 will secure the edge of theplate 27.1 to the adjoining portion of the housing to retain it in aprefixed location. The disc 27 will then be used simply by the I weareror the person who fits the hearing aid to the wearer to produce thedirectivity which is comfortable for the wearer to utilize.

It will be seen that this invention provides a hearing aid with adirectional microphone to receive the sounds from both front and rear,and'an adjustable acoustic resistance to vary the amount of resistanceto sounds being transmitted through one of the main access ports andinto the hearing aid to control the relative gain to sounds originatingfrom positions to the front of and to the rear of the hearingaid andthereby control the degree of directivity of the hearing aid. Althoughthe drawings illustrate the invention applied to a behindthe-ear hearingaid, the invention is also applicable to hearing aids incorporated ineyeglasses.

What is claimed is:

1. In a directional hearing aid, a microphone containing a diaphragm anda transducer and having soundreceiving openings in the front and rearportions of the microphone case to transmit sounds to both sides of thediaphragm and to create a significant acoustic resistance at the rearportion of the microphone case and thereby effect a phase shift of thesounds entering through such rear portions,

a hearing aid housing shaped to fit a persons head and oriented in afore and aft direction, the housing having a microphone chamber andfront and rear wall portions with sound access ports transmitting soundinto the chamber to the microphone, and

a variable acoustic resistance in one of said sound access ports andoperable to vary the gains relative to for covering and uncoveringportions of the grill to thereby change the acoustical resistance andvary the directivity of the hearing aid.

" 3. The'invention according to claim 1 and thevariableacousticalresistance having a movable part to be moved to varythe acoustical resistance, and said movable part having a controlportion accessible from the exterior of the hearing aid housing formoving said part to vary the directivity of the hearing aid.

4. In a directional hearing aid, a microphone containing a diaphragm anda transducer and having soundreceiving openings in the front and rearportions of the microphone case to transmit sounds to both sides of thediaphragm and to create a significant acoustic resistance at the rearportion of the microphone case and thereby effect a phase shift of thesounds entering through such rear portions,

a hearing aid housing shaped to fit to a persons head and oriented in afore andaft direction, the housing having a microphone chamber and frontand rear wall portions with sound access ports transmitting sound intothe chamber to the microphone, and

a variable acoustic resistance in one of said sound access ports andoperable to vary the gains relative to each other produce by the hearingaid of sounds originating, respectively, in front of and to the rear ofthe hearing aid and thereby change the directivity effect of the hearingaid, the acoustical resistance including a grill in the port, a maskingplate lying against the grill and movable along the grill for coveringand uncovering portions of the grill to thereby change the acousticalresistance and vary the directivity of the hearing aid, the maskingplate being rotatable and having alternate open and closed portionsadjacent each other circumferentially of the plate to cover and uncoverportions of the grill as the masking plate is rotated to progressivelyvary the directivity-of the hearing aid.

5. The invention according to claim 4 and the rotatable masking platehaving a rounded peripheral edge protruding through the housing to theexterior of the housing to facilitate manual rotation of the maskingplate to vary the acoustical resistance and directivity of the hearingaid.

6. The invention according to claim 4 and the grill having adjacentareas, one being partially open and substantially more previous topassage of sound therethrough than the other adjacent area, the closedportion of the masking plate moving across said partly open area of thegrill to vary the acoustical resistance and the directivity of thehearing aid.

7. The invention according to claim 6 and the rotatable masking platehaving a circular mounting hub fitting snugly in and rotatable withrespect to an opening in the grill, and said rotatable masking platelying flush against the grill to prevent sound from travelingtransversely between the grill and masking plate.

8. The invention according to claim 5 and the grill and masking platebeing disposed at the rear wall portion of the hearing aid housing andin the sound access port thereof to vary the resistance to soundstransmitted through the'rear port of the hearing aid and to themicrophone.

-9. The invention according to claim 5 and the grill and masking platebeing disposed at the front wall portion of the hearing aid housing andin the sound access port thereof to vary the resistance to soundstransmitted through the front portand to the microphone.

10. In a directional hearing aid, a microphone containing a diaphragmand a transducer and having sound-receiving openings in the front andrear portions of the microphone case to transmit sounds to both sides ofthe diaphragm and to create a significant acoustic resistance at therear portion of the microphone case and thereby effect a phase shift ofthe sounds entering through such rear portions,

a hearing aid housing shaped to fit to a persons head and oriented in afore and aft direction, the housing having a microphone chamber andfront and rear wall portions with sound access ports transmitting soundinto thechamber to the microphone, and

a variable acoustic resistance in one of said sound access ports andoperable to vary the gains relative to each other produced by thehearing aid of sounds originating, respectively, in front of and to therear of the hearing aid and thereby change the directivity effect of thehearing aid, the variable acoustical resistance having a movable part tobe moved to vary the acoustical resistance, and said movable part havinga control portion accessible from the exterior of the hearing aidhousing for moving said part to vary the directivity of the hearing aid,and a second variable acoustical resistance in one of the access portsto the hearing aid housing and having a manually operable controlportion disposed wholly within the housing to restrict access thereto.

11. In a hearing aid,

a microphone containing a diaphragm and a transducer and havingsound-receiving openings in the front and rear portions of themicrophone case to transmit sounds to both sides of the diaphragm, themicrophone having an acoustical resistance in the opening at the rearportions of the microphone case to delay thetransmission of soundsthrough the openings in the rear portions of the microphone and therebyinduce a directional effect in the microphone,

a hearing aid housing shaped to be fit against a persons head andoriented in a fore and aft direction, the housing having a microphonechamber and front and rear wall portions with sound access portstransmitting sound into the chamber to the microphone, the openings inthe front and rear portions of the microphone case being disposedadjacent to, respectively, the front and rear wall portions of thehousing with the front and rear access ports therethrough, means withinthe housing preventing transmittal of sound from the rear access port ofthe hearing aid housing to the front soundreceiving opening of themicrophone case and also preventing transmittal of sound through themicrophone chamber from the front access port and to the sound-receivingopenings at the rear of the microphone case,

a variable acoustic resistance in the sound access port in the rear wallportion of the hearing aid housing to change the directivity effect ofthe hearing aid, said variable acoustic resistance including astationary grill in the rear access port, the grill having adjacentareas of different sound transmitting capabilities, one of said areashaving a plurality of closely spaced and minute apertures defining clearpassages and cumulatively defining a substantial open area to provideminimum restriction and resistance to passage of sounds therethrough,said 5 individual apertures being sufficiently small as to provide verysubstantial resistance, acting individually and alone, to passage ofsound therethrough, the other of said areas adjacent to the aperturedarea having substantially less sound transmitting capability than saidapertured area, the periphery of the grill being sealed to the hearingaid housisng to prevent any passage of sounds around the edges of thegrill and into the microphone chamber, said acoustical resistance alsoincluding a rotatable masking plate in the rear sound access port of thehearing aid housing and lying flush against the grill, the periphery ofthe masking plate being disposed adjacent the periphery of the grill andpreventing passage of sounds therebetween, the masking plate havingalternate open and closed portions adjacent each other in a directioncircumferentially of the plate to cover and uncover portions of theapertured area of the grill, the rotatable masking plate having arounded edge portion protruding through the hearing aid housing and tothe exterior thereof to facilitate manual manipulation of the maskingplate which changes the acoustic resistance to sound entering throughthe rear port of the hearing aid housing to the microphone and changethe gains relative to each other produced by the hearing aid of soundsoriginating, respectively, in front of and to the rear of the hearingaid, thereby changing the directivity effect of the hearing aid.

12. The invention according to claim 11 and the rotatable masking platehaving an edge separating the open and closed portions thereof, saidedge extending from the central area of the masking plate outwardlytoward the edge thereof, and the apertured area of the grill having aboundary edge adjacent the other area of the grill and said boundaryedge having an orientation oblique to the edge of the open area of themasking plate whereby to progressively and incrementally close theapertured area of the grill as the masking plate passes thereover.

13. The invention according to claim 11 and there being a secondvariable acoustic resistance in the sound access port of the rear wallportions of the housing defining the microphone chamber, said secondvariable acoustic resistance having a grill with its peripheral edgeentirely engaging the housing and preventing passage of sounds into themicrophone chamber and around said peripheral edge, the grill of saidsecond acoustic resistance also having a plurality of minute soundtransmitting apertures defining clear passages into the microphonechamber and also having an area adjacent said apertures for obstructingpassage of sound through the grill of said second resistance, and thesecond resistance also having a movable masking plate lying flushagainst the grill and movable to progressively cover and uncover theapertures of the grill of the second sound transmitting resistance, andmeans confined within the housing for effecting movement of the maskingplate of the second acoustic resistance. 65

1. In a directional hearing aid, a microphone containing a diaphragm anda transducer and having sound-receiving openings in the front and rearportions of the microphone case to transmit sounds to both sides of thediaphragm and to create a significant acoustic resistance at the rearportion of the microphone case and thereby effect a phase shift of thesounds entering through such rear portions, a hearing aid housing shapedto fit a person''s head and oriented in a fore and aft direction, thehousing having a microphone chamber and front and rear wall portionswith sound access ports transmitting sound into the chamber to themicrophone, and a variable acoustic resistance in one of said soundaccess ports and operable to vary the gains relative to each otherproduced by the hearing aid of sounds originating, respectively, infront of and to the rear of the hearing aid and thereby change thedirectivity effect of the hearing aid.
 2. The invention according toclaim 1 and the acoustical resistance including a grill in the port, amasking plate lying against the grill and movable along the grill forcovering and uncovering portions of the grill to thereby change theacoustical resistance and vary the directivity of the hearing aid. 3.The invention according to claim 1 and the variable acousticalresistance having a movable part to be moved to vary the acousticalresistance, and said movable part having a control portion accessiblefrom the exterior of the hearing aid housing for moving said part tovary the directivity of the hearing aid.
 4. In a directional hearingaid, a microphone containing a diaphragm and a transducer and havingsound-receiving openings in the front and rear portions of themicrophone case to transmit sounds to both sides of the diaphragm and tocreate a significant acoustic resistance at the rear portion of themicrophone case and thereby effect a phase shift of the sounds enteringthrough such rear portions, a hearing aid housing shaped to fit to aperson''s head and oriented in a fore and aft direction, the housinghaving a microphone chamber and front and rear wall portions with soundaccess ports transmitting sound into the chamber to the microphone, anda variable acoustic resistance in one of said sound access ports andoperable to vary the gains relative to each other produce by the hearingaid of sounds originating, respectively, in front of and to the rear ofthe hearing aid and thereby change the directivity effect of the hearingaid, the acoustical resistance including a grill in the port, a maskingplate lying against the grill and movable along the grill for coveringand uncovering portions of the grill to thereby change the acousticalresistance and vary the directivity of the hearing aid, the maskingplate being rotatable and having alternate open and closed portionsadjacent each other circumferentially of the plate to cover and uncoverportions of the grill as the masking plate is rotated to progressivelyvary the directivity of the hearing aid.
 5. The invention according toclaim 4 and the rotatable masking plate having a rounded peripheral edgeprotruding through the housing to the exterior of the housing tofacilitate manual rotation of the masking plate to vary the acousticalresistance and directivity of the hearing aid.
 6. The inventionaccording to claim 4 and the grill having adjacent areas, one beingpartially open and substantially more previous to passage of soundtherethrough than the other adjacent area, the closed portion of themasking plate moving across said partly open area of the grill to varythe acoustical resistance and the directivity of the hearing aid.
 7. Theinvention according to claim 6 and the rotatable masking plate having acircular mounting hub fitting snugly in and rotatable with respect to anopening in the grill, and said rotatable masking plate lying flushagainst the grill to prevent sound from traveling transversely betweenthe grill and masking plate.
 8. The invention according to claim 5 andthe grill and masking plate being disposed at the rear wall portion ofthe hearing aid housing and in the sound access port thereof to vary theresistance to sounds transmitted through the rear port of the hearingaid and to the microphone.
 9. The invention according to claim 5 and thegrill and masking plate being disposed at the front wall portion of thehearing aid housing and in the sound access port thereof to vary theresistance to sounds transmitted through the front port and to themicrophone.
 10. In a directional hearing aid, a microphone containing adiaphragm and a transducer and having sound-receiving openings in thefront and rear portions of the microphone case to transmit sounds toboth sides of the diaphragm and to create a significant acousticresistance at the rear portion of the microphone case and thereby effecta phase shift of the sounds entering through such rear portions, ahearing aid housing shaped to fit to a person''s head and oriented in afore and aft direction, the housing having a microphone chamber andfront and rear wall portions with sound access ports transmitting soundinto the chamber to the microphone, and a variable acoustic resistancein one of said sound access ports and operable to vary the gainsrelative to each other produced by the hearing aid of soundsoriginating, respectively, in front of and to the rear of the hearingaid and thereby change the directivity effect of the hearing aid, thevariable acoustical resistance having a movable part to be moved to varythe acoustical resistance, and said movable part having a controlportion accessible from the exterior of the hearing aid housing formoving said part to vary the directivity of the hearing aid, and asecond variable acoustical resistance in one of the access ports to thehearing aid housing and having a manually operable control portiondisposed wholly within the housing to restrict access thereto.
 11. In ahearing aid, a microphone containing a diaphragm and a transducer andhaving sound-receiving openings in the front and rear portions of themicrophone case to transmit sounds to both sides of the diaphragm, themicrophone having an acoustical resistance in the opening at the rearportions of the microphone case to delay the transmission of soundsthrough the openings in the rear portions of the microphone and therebyinduce a directional effect in the microphone, a hearing aid housingshaped to be fit against a person''s head and oriented in a fore and aftdirection, the housing having a microphone chamber and front and rearwall portions with sound access ports transmitting sound into thechamber to the microphone, the openings in the front and rear portionsof the microphone case being disposed adjacent to, respectively, thefront and rear wall portions of the housing with the front and rearaccess ports therethrough, means within the housing preventingtransmittal of sound from the rear access port of the hearing aidhousing to the front soundreceiving opening of the microphone case andalso preventing transmittal of sound through the microphone chamber fromthe front access port and to the sound-receiving openings at the rear ofthe microphone case, a variable acoustic resistance in the sound accessport in the rear wall portion of the hearing aid housing to change thedirectivity effect of the hearing aid, said variable acoustic resistanceincluding a stationary grill in the rear access port, the grill havingadjacent areas of different sound transmitting capabilities, one of saidareas having a plurality of closely spaced and minute apertures definingclear passages and cumulatively defining a substantial open area toprovide minimum restriction and resistance to passage of soundstherethrough, said individual apertures being sufficiently small as toprovide very substantial resistance, acting individually and alone, topassage of sound therethrough, the other of said areas adjacent to theapertured area having substantially less sound transmitting capabilitythan said apertured area, the periphery of the grill being sealed to thehearing aid housisng to prevent any passage of sounds around the edgesof the grill and into the microphone chamber, said acoustical resistancealso including a rotatable masking plate in the rear sound access portof the hearing aid housing and lying flush against the grill, theperiphery of the masking plate being disposed adjacent the periphery ofthe grill and preventing passage of sounds therebetween, the maskingplate having alternate open and closed portions adjacent each other in adirection circumferentially of the plate to cover and uncover portionsof the apertured area of the grill, the rotatable masking plate having arounded edge portion protruding through the hearing aid housing and tothe exterior thereof to facilitate manual manipulation of the maskingplate which changes the acoustic resistance to sound entering throughthe rear port of the hearing aid housing to the microphone and changethe gains relative to each other produced by the hearing aid of soundsoriginating, respectively, in front of and to the rear of the hearingaid, thereby changing the directivity effect of the hearing aid.
 12. Theinvention according to claim 11 and the rotatable masking plate havingan edge separating the open and closed portions thereof, said edgeextending from the central area of the masking plate outwardly towardthe edge thereof, and the apertured area of the grill having a boundaryedge adjacent the other area of the grill and said boundary edge havingan orientation oblique to the edge of the open area of the masking platewhereby to progressively and incrementally close the apertured area ofthe grill as the masking plate passes thereover.
 13. The inventionaccording to claim 11 and there being a second variable acousticresistance in the sound access port of the rear wall portions of thehousing defining the microphone chamber, said second variable acousticresistance having a grill with its peripheral edge entirely engaging thehousing and preventing passage of sounds into the microphone chamber andaround said peripheral edge, the grill of said second acousticresistance also having a plurality of minute sound transmittingapertures defining clear passages into the microphone chamber and alsohaving an area adjacent said apertures for obstructing passage of soundthrough the grill of said second resistance, and the second resistancealso having a movable masking plate lying flush against the grill andmovable to progressively cover and uncover the apertures of the grill ofthe second sound transmitting resistance, and means confined within thehousing for effecting movement of the masking plate of the secondacoustic resistance.